The wire-coating agents conventionally used nowadays are solutions of wire enamel binders, such as, THEIC [tris(hydroxyethyl)isocyanurate]polyesters, polyesters, polyamides, polyamide-imides, THEIC polyester imides, polyester imides or polyurethanes in suitable organic solvents, such as, cresol, phenol, benzyl alcohol, propylene carbonate or N-methylpyrrolidone, as well as diluents, such as, xylene, other substituted aromatic substances, aliphatic substances and small additions of additives, catalysts and regulators. The solvents are evaporated during thermal curing of the wire coating agents. In order to obtain a high-quality coating, it is necessary to drive out the solvents as completely as possible. In addition to the solvents, by-products of the curing reactions pass from the enamelling phase into the gas phase as occurs during crosslinking by condensation reactions.
Selfbonding enamels (wire coating compositions) are usually used to bond, for example, individual electrically conductive wires together to form a compact and fixed wound item of the electrically conductive wires. For example, electrically conductive wires are usually coated by wire coating, for example, multi-layer wire coating known by a person skilled in the art. The selfbonding enamel is applied as a top enamel on the last cured coating layer of the multi-layer wire coating. After winding of the coated and top-enameled wire to be used, for example, in rotors, stators or transformers, and after heating the resulted wound item the enamel layer is melting and flowing between the windings. The windings are stuck together in order to fix the wound item and to maintain their function.
The selfbonding enamels contain, for example, polyamides, see, for example, U.S. Pat. No. 4,420,536 and U.S. Pat. No. 4,461,805. DE-A 3612372 describes a selfbonding enamel based on a polyamide which contains imide-functional groups. Such selfbonding enamels are thermoplastic systems, and are often limited because of their low re-softening temperatures and, therefore, of low fixing properties.
Also, thermosetting selfbonding enamels can be based on polyamides, but contain additionally curing agents. DE-A 19903137 discloses a composition with a highly reactive curing agent which needs the use of two-component formulations. Other compositions contain blocked isocyanates as curing agents, for example, CA 909990, DE-A 3517753, EP-A 461389 and DE-A 3917197. The release of the blocking agent during and/or after curing of the coatings may cause emissions and blistering effects and therefore, a decrease of the sticking power, or in case of non-volatile blocking agents a softening effect because of the remaining of the blocking agent in the cured coating. Also, such compositions may limit the re-softening temperature range of the selfbonding enamels.
DE-A 3903483 describes the use of bis-maleinimide as curing agent in selfbonding enamels which requires a high thermal energy.